My second time around my monarch married an NPC and of course, he vanished like the others. However, once she finally got him to woohoo with her and they had a child he was usually in or around the castle and spent a good amount of time with the baby. Once the baby aged up he still stayed with her though their interactions were minimal. They bill and coo and do kissy face a lot. This was with the "handsome lass".
First time around with a different monarch she wound up having to get a divorce since she could never pin the dude down long enough for a woohoo and baby. She was seriously considering the pit.
Third time it was the heir to Effonmount. She was in love with Lassiter, (again), but had to break up with him, (and he got quite nasty over it!), in order to do her duty to the throne. Marrying the heir to Effonmount was a huge mistake. So far she can't pin him down to woohoo and produce an heir to either throne! In the future that would be a quest I would avoid doing.
In my current game I have married my merchant to my knight, both drunkards and very happy, (he will be my choice for my next kingdom's king), my physician to my bard, they have one child, and I'm considering what other hookups I want. Blacksmith and spy? Peteran and wizard?